Inductive position sensor

ABSTRACT

A detector to measure the displacement of relatively moveable bodies along an axis comprising: a resonant electrical intermediate device further comprising an inductor, whose width varies along the displacement axis, and a capacitor in electrical series which co-operates with an antenna comprising transmit and receive windings whose mutual inductance varies according to the position of the electrical intermediate device relative to the antenna.

FIELD OF THE INVENTION

This invention relates to an inductive displacement detector, operableto measure the displacement of relatively moveable bodies and which hasparticular, but not exclusive, relevance to motor encoders.

REVIEW OF THE ART KNOWN TO THE APPLICANT

Various forms of detector have been used to measure the position of tworelatively moveable bodies. They are variously referred to as detectors,sensors, transducers, encoders, measurement devices or systems.

A common form of encoder is the optical encoder. This type of encodertypically uses an optical grating which moves relative to an opticaltransmitter and receiver. Such devices are widely available fromcompanies such as Baumer Electric Ltd., Shrivenham Hundred BusinessPark, Watchfield, Wiltshire, England. As with most optical systems, anyingress of foreign matter such as dirt or grease can cause obscurationof the optical sensor. This can cause failure with little or no warning.Further, such optical systems require very fine optical gratings forhigh resolution measurement. These fine gratings exacerbate thesusceptibility to foreign matter, as well as being difficult andexpensive to manufacture.

A more robust alternative to the optical form of encoder is the magneticencoder. These are also available from companies such as Baumer ElectricLtd., but typically do not provide high measurement resolution comparedto optical systems. Magnetic encoders require precision housings andmechanical assembly to avoid errors caused by magnet or sensormisalignment. Further problems include susceptibility to strayelectromagnetic fields and batch to batch variability of the magnetscaused by differences in material composition.

Patent GB 1 414 517 discloses an inductive transducer in which a rotorco-operates with a stator. The rotor carries a castellated windingarrangement and the stator carries a spiral transmit winding and anarrangement of two castellated receive windings Application of a highfrequency input to the transmit winding results in a modulated outputfrom the receive windings which may be demodulated to provide a signalindicative of the rotor's position relative to the receive windings. Thewinding arrangements are embodied on single layer printed circuit boardand there is no opportunity for the use of electromagnetic balancing.Consequently, the received signals are small and the signal:noise ratiolow. Without comprehensive shielding the device would be highlysusceptible to electromagnetic disturbances. In order to get the deviceto work, the location and arrangement of the rotor relative to thestator would need to be very precise and, in particular, the arrangementrequires a small distance between the faces of the rotor and stator.

U.S. Pat. No. 4,737,698 discloses an inductive encoder in which aconductive target, such as a copper disk, moves relative to a planararrangement of transmit and receive windings. Application of a highfrequency input to the transmit winding results in a modulated outputfrom the receive windings which may be demodulated to provide a signalindicative of the target's position relative to the receive windings. Inorder to produce sufficiently high signal to noise ratios, a relativelyhigh input voltage signal is required and the target's stand offdistance must be kept to a minimum. High input energy produces highelectromagnetic emissions and the minimal stand-off distance limits thescope of application. Further, the position of only one target may bedetected per set of transmit and receive windings.

U.S. Pat. No. 5,796,250 discloses a rotary displacement encoder in whichvarious rotors each contain an electrically resonant, passive circuit.The rotors are housed in a 3-dimensional arrangement of transmit andreceive windings. The invention in U.S. Pat. No. 5,796,250 solves someof the problems normally associated with traditional encoders orinductive encoders with conductive targets but the winding arrangementis only suitable for rotary or odometer types of construction.

The authors have previously disclosed a number of inventions relating tothe measurement of electrical intermediate devices which move relativeto planar arrangements of transmit and receive windings. For example,GB02/01204, discloses a method in which high frequency carrier signalsare modulated with lower frequency signals in order to provide a digitalsignal generation and processing technique suitable for positionmeasurement. The invention has limited application in position encodingbecause of its relatively low speed signal generation and processingalgorithm. It is not generally suitable for applications such as motorencoders which require accurate and rapid position measurement at speedsof >10000 rpm.

Multi-layer printed circuit boards (PCBs) are a useful constructiontechnique for antenna and electrical intermediate devices in encoderslike those described in GB02/01204 and U.S. Pat. No. 4,737,698. However,a relatively large number of inter layer plated through holes or vias(typically >25) are required to form the cross overs in the conductivetracks of the antennae when produced using PCB techniques. These viaholes distort the antenna's electromagnetic field and the high numberrequired for multi-pitch designs, especially in small encoders, isproblematic in designing high resolution measurement systems with goodlinearity (typically <1%). The vias tend to limit the smaller sizes atwhich such constructions may be deployed. These problems areparticularly acute given the increasing use of small DC brushless motorsin products such as cameras, consumer electronics and computercontrolled devices where the permissible space envelope for a motorencoder can be <30 mm diameter and often <20 mm.

The present invention encompasses the concept of a low cost, high speed,accurate and robust device to detect the relative positions of two ormore bodies which is applicable to a variety of topologies but hasparticular utility in motor encoders.

In a preferred embodiment, the device comprises an arrangement oftransmit and receive windings attached to a first body wherein at leastone of the transmit and receive windings is formed from a serpentineconductor extending along the measurement axis substantially on oneplane and returning substantially on a different plane so as to formloops such that voltages induced in adjacent loops oppose each other andat least one electrical intermediate device attached to a second bodyoperable to move along an axis relative to the first body and whereinthe electrical intermediate device comprises a capacitor and inductor inelectrical series and wherein the width of the inductor varies such thatthe level of mutual inductance between the transmit and receive windingsvaries according to the position of the two bodies.

SUMMARY OF THE INVENTION

In its broadest aspect, the invention provides a device for measuringthe position of a first body relative to a second body comprising: afirst body which further comprises an antenna arrangement of transmitand receive windings and a second body which comprises an electricallyresonant intermediate device whose inductor width, measured at rightangles to the measurement axis, varies such that the level of mutualinductance between the transmit and receive windings varies according tothe position of the two bodies.

Viewed from a further aspect, the invention provides an inductiveposition detector as defined in claim 1.

Preferably at least one of the receive windings is arranged as a seriesof loops such that voltages induced in adjacent loops oppose each other.

Preferably at least one of the receive windings is formed from aserpentine conductor which extends along the measurement axissubstantially on one plane and returns substantially on a differentplane so as to form a series of loops.

Preferably the receive windings are arranged side by side along themeasurement path and do not overlap.

Preferably the receive and transmit windings are arranged on a singleprinted circuit board so as to form an antenna.

Preferably the receive windings are displaced relative to each otheralong the measurement path by a quarter winding pitch so as to form asine and cosine winding.

Preferably the electrical intermediate device comprises a capacitor andinductor in electrical series so as to form a resonant or tank circuit.

Preferably the electrical intermediate device's inductor is formed by aconductive track on a printed circuit board and whose width variesperiodically along the measurement axis.

Preferably the electrical intermediate device's inductor is formed by aserpentine conductor extending along the measurement axis on a plane andreturning on the same plane.

Preferably the windings in the electrical intermediate device's inductorand the receive windings are in a multi-pitch periodic arrangement so asto provide high resolution but ambiguous measurement over themeasurement range.

Preferably a multi-pitch periodic electrical intermediate device andantenna co-operate with a single pitch electrical intermediate deviceand antenna to provide high resolution, absolute position measurementover the measurement range.

Preferably the detector is controlled by an electronics circuit whichhas a first 2-bit digital quadrature electrical output indicative ofcoarse but high speed position and a second multi-bit digital dataoutput indicative of high resolution but low speed position.

Preferably the electronics circuit comprises a microcontroller withmemory which may store data such as calibration values.

Preferably the detector is calibrated, prior to use, with a measuringinstrument of higher measurement accuracy and the resulting calibrationvalues stored in the detector's electronic memory.

Preferably the detector is controlled by an electronics circuit which isconstructed so that it may control several sets of detectors each ofwhich has its own distinct resonant frequency.

Preferably the detector is controlled by an electronics circuit which isconstructed so that it may control several sets of detectors each ofwhich has its own period of time in which to transmit and receivesignals.

Preferably the antenna comprises electrically balanced transmit andreceive windings.

Preferably the antenna comprises sets of receive windings of differentpitches so as to form a Vernier pattern.

Preferably the electrical connections to the antennae or electronics aretransmitted via a set of transformer windings.

Preferably the electrical intermediate device's electrical circuitcontains a switch in series with the capacitor and inductor so as toprovide contactless transmission of switch status.

Preferably the electrical intermediate device contains a multiplicity ofresonant circuits whose frequency or position or both may be detected soas to provide a method of identification.

Preferably an electrical intermediate device whose position is fixed orknown is used together with at least one electrical intermediate devicewhose position is to be measured so that the reliable operation of thedetector may be checked.

Preferably the electrical intermediate device's inductor has amagnetically permeable core which is operable to displace relative topart of the inductor's windings so that the change in the core'sposition alters the inductance value which, in turn, alters theelectrical intermediate device's resonant frequency and hence permitscontactless detection.

Preferably the detector is used to measure the relative twist betweentwo parts of a stationary or rotating shaft as a means of measuring thetorque applied to the shaft.

Preferably the detector's antenna in a rotary embodiment comprises atleast one winding which occupies a segment of substantially less than360 degrees.

Preferably the detector compensates for temperature variation bymeasuring temperature and altering the measured position values.

Preferably the detector measures temperature by measuring the resistanceof at least one of the antenna's windings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings;

FIG. 1 shows a schematic form of a known inductive detector.

FIG. 2 shows a simplified, schematic linear form of the detector.

FIG. 3 shows an electrical schematic of the electrical intermediatedevice's resonant circuit.

FIG. 4 shows a schematic arrangement of the detector's electroniccircuit.

FIG. 5 shows a Vernier arrangement of windings for the electricalintermediate device's inductor.

FIG. 6 shows a triangular form of the antenna.

FIG. 7 shows a linear form of the detector with a transformer couplingacross a membrane.

FIG. 8 shows the detector as a rotary user interface element with apush-button switch.

FIG. 9 shows an electrical schematic of the electrical intermediatedevice shown in FIG. 9.

FIGS. 10 a, 10 b, 10 c, 10 d, 10 e & 10 f show various arrangements ofelectrical intermediate device so that they may be used as a method ofidentification.

FIG. 11 shows part of an electrical intermediate device's inductor witha magnetically permeable core which is operable to displace relative tothe inductor.

FIG. 12 shows a schematic of a system with multiple sensors controlledby a single electronic circuit.

FIG. 13 shows an arrangement for measuring torque in a shaft which usesa transformer winding to energise and receive signals from the antenna.

FIG. 14 shows an arrangement for measuring torque in a shaft.

FIG. 15 shows an antenna winding arrangement suitable for rotaryposition measurement.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a simplified, schematic, plan view of a known form ofposition encoder. An antenna [1] contains a sinusoidal receive winding[1 a] and a cosinusoidal receive winding [1 b], surrounded by a transmitwinding [1 c] co-operating with an electrical intermediate device [2].The two receive windings [1 a & 1 b] are displaced by ¼ of the windingpitch along the measurement (x) axis. The two receive windings [1 a & 1b] are arranged in series of loops along the measurement axis such thatvoltages induced in adjacent loops oppose each other. The loops in thereceive windings [1 a & 1 b] are formed using via or plated throughholes positioned on either side of the measurement axis. It should benoted that in this much simplified example only 8 vias are shown forreasons of clarity. However, in practice, each of the receive windings[1 a & 1 b] is likely to use multiple turns in each loop andconsequently many more than 8 vias will be required. >100 vias may berequired if a multi-pitch arrangements is used in which each loop hasmultiple turns. All the windings are arranged along an axis [x] on aplanar, insulating substrate over a full scale [L]. The antenna [1]co-operates with an electrical intermediate device [2]—such as a copperdisk. The transmit winding [1 c] generates an alternatingelectromagnetic field when energised with an AC voltage. When theelectrical intermediate device [2] enters the field, currents areinduced to flow in it which, in turn, produce their own electromagneticfield. The ratio of the voltages induced in the receive windings [1 a &1 b] is indicative of the electrical intermediate device's positionrelative to the antenna. This type of encoder works well but isgenerally only suitable to applications in which measurement frequenciesof <500 Hz are acceptable. This is due to the need to sample receivewinding voltages, ratio mathematically and then digitise rotor position.

Furthermore, the large number of PCB via holes further limits suchinventions to relatively large constructions with relatively fewpitches. It would be impossible, for example, to arrange such aconstruction (with commercially attractive measurement characteristics)in a 20-pitch rotary embodiment on a 15 mm diameter rotor and statorusing traditional printed circuit board techniques. Far fewer pitcheswould be possible with traditional constructions but inductive encoderswith relatively few pitches have relatively low resolution.

FIG. 2 shows a simplified, schematic, plan view of the invention's mainelements in which a resonant electrical intermediate device [2]co-operates with an antenna [1]. For reasons of clarity the electricalintermediate device [2] is shown to one side of the antenna [1],displaced in the y-axis. In operation, the electrical intermediatedevice [2] would be located directly above the antenna [1]. In apreferred embodiment the electrical intermediate device [2] and antenna[1] are used to detect the relative displacement of the bodies to whichthey are attached. The bodies are arranged so as to move principally ina single linear axis (x) although it is important to note that theinvention is not restricted to linear motion. The antenna [1] iselectrically connected to an electronic circuit [not shown for clarity].The electronics circuit [18] is energised by an electrical power supplyand outputs electrical data in accordance with the relative position ofthe electrical intermediate device [2] and antenna [1]. The antenna's[1] transmit winding [1 c] is energised with an AC signal by theelectronics circuit [18] at substantially the same frequency as theresonant frequency of the electrical intermediate device [2]. When theelectrical intermediate device [2] enters the field produced by theenergised transmit winding [1 c], currents are induced to flow in theelectrical intermediate device [2] which, in turn, cause their ownelectromagnetic field. This field can be sensed by the antenna's receivewindings [1 a & 1 b]. The antenna's receive windings [1 a & 1 b] arelaid out in a multi-pitch fashion and are displaced side by side to eachother in the y-axis and shifted relative to each other along the x-axisby ¼ of their winding pitch so as to form a sinusoidal and cosinusoidalwinding. In this particular embodiment their footprints have zerooverlap (although some degree of limited overlap is permissible in otherarrangements to minimise the amount of PCB area required—as describedlater in FIG. 6). Each of the antenna's receive windings [1 a & 1 b] isarranged as a series of loops extending along the measurement path andformed by a first or outward serpentine conductor on the top layer and areturn serpentine conductor on the bottom layer. Connection of theoutward and return serpentine conductors is made at a single via hole[3] at the end of the measurement path. The resonant electricalintermediate device [2] is constructed in a similar but differentmanner. Preferably the resonant electrical intermediate device'sinductor [6] is formed by a conductor extending along the measurement(x) axis in a periodic serpentine pattern on a plane and returning onthe same plane back along the measurement axis so as to form a loopwhose width varies periodically along the measurement axis. Preferablythe period of the electrical intermediate device's inductor [6] issubstantially the same as the period of the receive windings [1 a & 1b]. Preferably the transmit winding [1 c] is arranged in electricalbalance with respect to the receive windings [1 a & 1 b]. As theelectrical intermediate device [2] displaces relative to the antenna [1]the voltages induced in the receive windings [1 a & 1 b] varysinusoidally. When the voltage is maximised in the sine receive winding[1 a] the signal is minimised in the cosine receive winding [1 b]. Twovoltages in quadrature are received. These can be measured and anarctangent calculation carried out to provide an electrical indicationof the relative displacement. Clearly, the position is absolute overeach pitch of the windings but ambiguous over the full scale. Electricaloutputs from the electronic circuit are proportional to the position orchange in position of the electrical intermediate device [2] relative tothe antenna [1]. A power supply of 5 VDC with a current of 10 mA ispreferable. Preferably the electrical intermediate device [2] ismanufactured from a single sided printed circuit board such as CEMI with1 ounce copper tracking. Preferably the antenna [1] and correspondingelectronics circuit [18] are constructed from a double sided printedcircuit board such as 0.8 mm thick FR4 with 1 ounce copper tracking.Preferably the printed circuit boards are protected from humidity andfluids with a sprayed conformal coating such as Plastic Seal 60 fromServisol Ltd. of Bridgwater, Somerset, England.

It will be obvious to those skilled in the art that the linear formdescribed above can be transformed into a rotary version by wrapping theends of each of the various windings around to complete a circle. Inthis case it is preferable to adjust the effective width of each receivewinding [1 a & 1 b] to achieve an equal coupling area. Indeed animportant commercial application for the invention is in a rotary formfor motor encoders but the linear form is described here for reasons ofclarity and ease of understanding. Measurement linearity may also beimproved by varying the mark space of the serpentine outward and returnconductors.

FIG. 3 shows an electrical schematic of the target's resonant circuit,which comprises an inductor [6] and capacitor [7] in electrical series.Preferably resonant frequencies in the range 100 kHz-10 MHz are used.

FIG. 4 shows a schematic of the electronic circuit which is largely selfexplanatory. Since the signal in first receive winding RXa isproportional to sin(x) and the signal in the second receive winding RXbis proportional to cos(x), then position ‘x’ can be obtained from theequation (L/2pi)*ARCTAN(RXa/RXb). The power supply may pass through anover voltage protection circuit if voltage spikes are possible from theelectrical supply. Reverse polarity protection is also a commonrequirement. Typical power requirements are 5V at 10 milliamps at a 100%duty cycle. This may be reduced generally in line with the reduction induty cycle through a sleep mode. Preferably, the output signal for highbandwidth ‘encoder’ mode is in a pulse train. Preferably, the outputsignal for the lower bandwidth ‘analogue’ mode is a serial data streamsuch as SPI. Preferably the circuit [18] is constructed either from anapplication specific integrated circuit or a circuit of surface mountelectronics soldered to the same printed circuit board as the antenna[2] with a short distance between the circuit [18] and the antenna's [2]windings so as to avoid any electromagnetic emissions or susceptibilityproblems. One aspect of the circuit [18] worthy of note is that thecircuit has two modes of operation that can run independently, ifrequired. In the first mode of operation—referred to as ‘encoder’mode—two pulse trains are produced—as used already as a defacto standardin many of the existing optical or magnetic proprietary encoders. Thepulses are produced by detecting the effective zero crossings of thesine and cosine signals produced by the two receive windings. This modehas the advantage of high bandwidth and is of particular utility forrelatively coarse position measurement at high speeds. Such a mode maybe used in electrical commutation of motor windings for example or forspeed control at high speed. In the second mode of operation—referred toas ‘analogue’ mode—two sinusoidally varying analogue voltages areproduced in quadrature. From these signals, position may be calculatedaccurately by using an arctan calculation. This mode has the advantageof high-resolution position measurement. It has particular utility infine position measurement at lower speeds. Such a mode may be used inthe alignment of mechanical parts driven by a motor. The two regimes ofhigh speed pulse counting, or rate measurement, combined with low speedfine position measurement are a common requirement of motor controlsystems. For the ‘encoder’ mode to operate well (independently of the‘analogue’ mode) the maximum number of pitches should be formed alongthe available measurement path. This can be achieved as described aboveby using a winding layout which requires very few via holes in theelectrical intermediate device and the receive winding layouts.

Modifications and Further Embodiments

In FIG. 2 an electrical intermediate device [2] is shown with arectangular form serpentine conductor co-operating with rectangularformed windings [1 a & 1 b] on the antenna [1]. The invention is notnecessarily constrained to such a construction. The windings may be in asinusoidal, triangular, elliptical or other such form. FIG. 6 shows atriangular form of the winding arrangement in the antenna [1] embodiedon a PCB. The antenna [1] comprises a transmit winding [1 c] extendingalong the measurement axis and surrounding two receive windings [1 a & 1b] which are offset by ¼ of their winding pitch so as to form asinusoidal and cosinusoidal winding. Each of the receive windings [1 a &1 b] is formed by a serpentine conductor extending along the measurementaxis on the top layer of a PCB; connecting to the bottom layer via aplated through hole towards the end of the antenna [1] and returningback in a serpentine fashion so as to form loops in each of the windingssuch that voltages induced in each of the adjacent loops oppose eachother

FIG. 2 shows a multiple pitch arrangement. Such arrangements oftypically >6 pitches are preferable for many of the invention'scommercial applications—notably motor encoders. The invention is notnecessarily constrained to multiple pitches and may comprise just asingle pitch electrical intermediate device and antenna.

So far, an arrangement with a single transmit and two receive windingshas been described. The invention is not necessarily so constrained andcan operate with transmit windings taking the place of the receivewindings described thus far and a single receive winding taking theplace of the transmit winding described thus far. Essentially theinvention described thus far can be run in reverse with time-phaseshifted signals fed in to the spatially phase shifted windings thanks tothe principle of mutuality.

In FIG. 2 an electrical intermediate device [2] with an inductor [6] isshown with a serpentine conductor extending along the x-axis (i.e. theoutbound) with a return conductor back to the capacitor in a straightline. The invention is not necessarily constrained to such aconstruction and the return conductor may be returned on the other sideof the PCB in a similar fashion to the outbound conductor. The mainaspect of the inductor [6] is that its width should vary periodicallyalong the measurement (x) axis.

The length of the electrical intermediate device [2] does not have to beas long as the antenna [1] as described thus far. Whilst this ispreferable for small diameter rotary encoders it is not preferable forrelatively long linear displacements where either a long electricalintermediate device [2] should be paired with a shorter antenna [1] orvice verca.

Other spatial harmonic width variation components may be added to thefundamental varying part of the windings to improve linearity.

Measurement resolution over a distance L can be improved by the use ofmultiple pitch windings as shown in FIG. 2 compared to a single pitcharrangement. If, for example, an electronics circuit [18] producesmeasurement resolution of 10 bits over a full scale of L in a singlepitch arrangement, this can be improved by arranging the electricalintermediate device's and antenna windings in to multiple pitches of <L.For example, if the windings are arranged in 8 pitches each of lengthL/8 then the measurement resolution will be equivalent to 10 bits overL/8, in other words a 8-fold improvement in measurement resolution.

One draw back of multi-pitch arrangements is that the measured positionis ambiguous rather than absolute over the full measurement distance.Absolute position measurement can be achieved by the use of a second,coarse pitch arrangement extending over the full scale. Preferably anelectrical intermediate device of a second frequency co-operates with anarrangement of transmit and receive windings as described in theauthor's patent application GB0427410.6. In this way a high resolution,ambiguous measurement can be produced in addition to a low resolution,absolute measurement. The two measurements may be combinedelectronically or in software to produce a high resolution, absoluteposition measurement. The same frequency can be used for both electricalintermediate devices if there is sufficient spatial separation betweenthem.

A first alternative scheme to coarse and fine pitch winding arrangementsis the use of a Vernier technique. A schematic of such an arrangement isshown in FIG. 5. This technique uses two or more multi-pitch windings inthe antenna For example, a first and second receive winding [1 a & 1 b]of pitch L/6 is used with a third and fourth receive winding of pitchL/7 [1 aa & 1 bb]. There are various ways to construct the correspondingelectrical intermediate device [2]. A first method is to use a singleinductor [6] pitch winding of L/6.5. A second method is to use two tankcircuits the first with an inductor [6] of pitch L/6 cooperating withthe L/6 receive windings and a second inductor [6]—at a differentfrequency—of pitch L/7 cooperating with the L/7 receive windings. Athird method is to use an electrical intermediate device inductor [6] ofpitch L/6 which extends to cover both sets of L/6 & L/7 receive windingsbut which is <6 pitches long. Whatever construction of electricalintermediate device [2] is used, the ambiguous readings can be combinedso as to provide a unique or absolute indication of position over themeasurement range. The unique indication will be preserved up to thelowest common multiple of the winding pitches. In this example thelowest common multiple is L.

A second alternative to enable high resolution measurement over extendedscales can be achieved with the use of a reed or Hall switch forexample. In such an instance a magnet is attached to the electricalintermediate device which triggers the reed or Hall switch to signifythat, for example, a second area of the electrical intermediate deviceis in operation.

A third alternative to enable high resolution measurement over extendedscales can be achieved with the use of a repetitive winding pitch over along distance (with fundamentally incremental position measurement) butwherein the electronic circuit counts the number of cycles so as toprovide an absolute electrical signal. This count can be checked againsta single point resonant device at a second frequency placed along themultiplicity of repetitive windings. When the antenna [2] andelectronics circuit [18] passes over the shorter pitch resonant devicethe count may be checked and rectified in software if necessary.

FIG. 7 shows a linear form of the encoder in which the electricalconnections [10] to the antenna cross a membrane [9] withoutmechanically perforating the membrane. The arrangement has particularuse in pressure vessels or vessels containing liquids where perforationsor holes in the vessel are preferably avoided. The electrical cables[10] are each connected to a set of primary windings [11] of atransformer on the first side of the membrane [9]. The electricalsignals are carried across the membrane to the transformers secondarywindings [8] on the second side of the membrane. Preferably thetransformer windings [8 & 9] are embodied as spiral tracks on a printedcircuit board.

FIG. 8 shows a rotary user element [12] with a push button [13]. Such anarrangement might be used, for example, in a user interface for aninformation system so that a user might scroll through a menu structurein a graphical user interface and make a selection. The user element[12] is operable to rotate under the action of a user. The user element[12] contains a rotary electrical intermediate device [2] whose positionis measured by an antenna (not shown) positioned on the underside of animpermeable fascia panel [15]. The electrical intermediate device [2]also contains a push button [13] and a switch [14] which is normallyopen but closed under the action of a user depressing the push button[13]. Such an arrangement is advantageously deployed when thetransmission of switch status is required to be communicated in anon-contact fashion, for example, through an impermeable membraneproviding a hermetic seal. Advantageously the user element [12] may beheld against the fascia panel [15] under the action of two or moremagnets—the first in the user element [12], the second in or beneath thefascia panel [15]. In such an arrangement the user element [12] may bereadily removed so that a smooth impermeable fascia is left tofacilitate ease of cleaning or sterilisation and during which no ingressof fluid to the host equipment is possible. Further user interactionwith the host equipment is prevented which is useful in preventingaccidents caused by improper use by children, vandals or untrainedpersonnel. Additional magnets may be used to provide tactile feedback asthe user element [12] is operated.

FIG. 9 shows an electrical schematic of a non-contact method oftransmitting switch status as used in the system described in FIG. 8. Anormally open switch [14] is arranged in electrical series with theelectrical intermediate device's capacitor [7] and inductor [6]. When alocal antenna [1—not shown] is energised, currents will be unable toflow in the electrical intermediate device [2] because of the opencircuit. No return signal will be received by the antenna [1]. When theswitch [14] is closed, currents will flow and the antenna [1] willreceive a signal. In this way it is possible for data regarding switch[14] position to be transmitted in a contactless manner. Data regardingmultiple switches [14] may be provided by using multiple resonantcircuits of different resonant frequencies. Alternatively, the switches[14] can be used to switch in other capacitors [7] or inductors [6] soas to alter the natural frequency of the electrical intermediate device[2] and hence provide data on switch [14] position. Alternativelynormally closed switches may be used and the loss of signal when theswitch is depressed sensed accordingly.

FIGS. 10 a, 10 b & 10 c show schematic methods of encoding identities onto electrical intermediate devices.

In FIG. 10 a an inductor [6] and capacitor [7] are arranged inelectrical series to form an electrical intermediate device [2]. Varyingeither the capacitor [7] or inductance [6] will vary the resonantcircuit's natural frequency. This can be used as a way of encodingidentity against a predetermined set of identities and frequencies heldin the electronic circuit's memory. Typically, a practical range ofpossible frequencies will be limited to about 10, so that inherentfrequency variations can be tolerated and frequency harmonics avoided.

In FIG. 10 b 2 resonant circuits are provided. In this way thecombination of potential resonant frequencies increases the practicalnumber of possible identities. Further by varying the distance betweenthe circuits, p, this non-frequency based variable increases the rangestill further. Large numbers of identities can be encoded with the useof more than 2 electrical intermediate devices [2] and 2-dimensional (inx & y-axes) sensing of relative displacement.

FIG. 10 c shows a schematic of a resonant circuit with inductors inparallel. By obliterating part of the winding of one or more of theinductors [6] at positions marked X then the circuit's inductance isaltered and, in turn, the resonant frequency. This can be readilyperformed using a computer numerically controlled printed circuit boarddrilling machine and is an alternative method of varying a circuit'sfrequency compared to changing the value of the capacitor. In this way,resonant circuits can be produced in bulk from a small number of PCB'sand capacitors and then tailored to requirements at point of delivery.

FIG. 10 d shows the use of 2 rotary electrical intermediate devices [2]whose centre distance is constant but whose rotary position can bevaried. By way of example, we may consider the use of 2 electricalintermediate devices [2] of the same frequency whose rotational positioncan be measured by 2 electrically separate antennae [1]. If each of therotary electrical intermediate devices [2] can be set to one of 36positions then the total number of identities=36×36=1296.

FIG. 11 shows an alternative arrangement to provide contactlesstransmission of data other than making or breaking a switch [14]contact. In some applications it may be undesirable to use an electricalswitch [14]. For example in wet or dirty environments the switch [14]may become unreliable due to foreign matter or fluid ingress at theswitch's [14] electrical contacts. In such instances data may beprovided by changing the inductance value of part of the inductor [6] inthe electrical intermediate device [2]. FIG. 11 shows part of theelectrical intermediate device's inductor [6] which has been wound in toa coil [16]. A magnetically permeable element [17]—preferably a ferritecore—is arranged so that, if it is pressed, it moves along the main axisof the coil [16] and causes a change in inductance. This will bedetected as a shift in the resonant frequency of the electricalintermediate device [2]. Preferably the coil [16] is placed in serieswith the electrical intermediate device's inductor [6].

FIG. 12 shows a schematic of a network of antennae [1] and electricalintermediate devices [2] of various shapes and sizes. Each of theantennae [1] is energised by a single electronics circuit [18] via amultiplicity of interconnecting conductors [10]. Each of the antennae[1] and electrical intermediate device [2] sets have their own resonantfrequency, for example 1 MHz, 2 MHz and 3 MHz, and the excitations,signal receptions and resulting position calculations for each detectorcan be made as the excitation frequency steps through a pre-programmedsequence. Alternatively time division multiplexing of the transmit andreceive connections between antenna [1] and electronics circuit [18] ata single frequency can be employed.

FIG. 13 shows a sectional schematic of the detector's main elementsco-operating with a rotating shaft [19] so that the degree of twistbetween points A and B may be measured. Advantageously such ameasurement is an indirect method of measuring applied torque. Anelectrical intermediate device [2] is attached to point B on the shaftusing a bar [22], collar [20] and a grub screw [21]. An antenna [1]comprising a planar arrangement of transmit and receive windings [1 a, 1b & 1 c] is attached to the shaft [19] at point A. As a torque isapplied to the shaft [19] the angular position of the shaft's [19]circumference at point A shifts slightly relative to the angularposition of the shaft's [19] circumference at point B. The magnitude andsense of the angular variation is directly proportional to magnitude andsense of the applied torque up to the shaft's [19] elastic limit. Themagnitude of the angular variation may be increased by providing slotsor grooves in the shaft [19] as shown in FIG. 13, in order to provide asufficiently large angular variation that might be measured accurately.If the torque variation produces a maximum angular variation of ±Xdegrees then preferably the antenna [1] and electrical intermediatedevice [2] are arranged in a multi-pitch construction of 360/2X pitches.The transmit windings [1 c] are energised from an electrical supply viaan annular transformer [23] whose central hole fits over the shaft [19].The signals from the antenna's receive windings [1 a & 1 b] aretransported using multiple windings of the same transformer [23] to anelectronics circuit [18—not shown for clarity]. Preferably the axialdistance between the transformer [23] and the antenna [1], as well asthe distance between the electrical intermediate device [2] and theantenna [1] should be kept as small as possible to produce good levelsof electrical coupling. Good electrical coupling produces good signal:noise ratios in the detector.

FIG. 14 shows an alternative arrangement for the measurement of torque.Rather than a transformer [23] two electrical intermediate devices [2]are used on either side of the antenna [1]. The position of both may bemeasured as the shaft [19] rotates such that their relative shift isindicative of applied torque.

FIG. 15 shows an antenna [1] arrangement suitable for rotarymeasurements. In this arrangement the receive windings [1 a & 1 b]occupy only a segment or portion of the antenna rather than running afull 360 degrees. Such an arrangement is space efficient and isparticularly advantageous for encoders in small motors. Further, themaximum stand off distance between target [2] and antenna is increasedfor a given antenna size.

The antenna's transmit and receive windings [1 a, 1 b & 1 c] may besimplified when the invention is used in a pulse echo mode. In this modethe windings are first of all energised with an AC signal which is thenswitched off to allow the windings then to receive any signals comingback from a resonating electrical intermediate device. Only 2 windingswith known separation are required.

There is no absolute size limitation to the invention. The limits areset by limits of manufacturing processes rather than physical laws. Atone extreme, very large detectors can be produced by winding copper wireover pegs arranged at defined positions on a surface. At the otherextreme, very small detectors can be produced using deposition ofconductive tracks on a silicon wafer. This is particularly advantageouswhen the tracks are deposited on the same silicon as that used for theelectronics circuit [18] in the form of an application specificintegrated circuit. The use of conductive inks printed on to aninsulating substrate such as polyester or polyamide are particularlyuseful technique to produce electrical intermediate devices [2] andantennae [1]. Advantageously, polyester and polyamide substrates may beproduced in a flexible form which may also be printed with a contactadhesive for ease of attachment to a host system. In printedconstructions electrical insulation at cross over points can bemaintained by printing a first conductive track followed by aninsulating layer at the area of the cross over and then anotherconductive track over the top of the insulating layer. Glass is aparticularly good substrate in harsh environments due to its stabilityand low coefficient of thermal expansion. Further construction methodsinclude double sided printed circuit board with or without platedthrough holes; ultrasonic bonding of insulated copper wire on to aninsulated substrate and windings which are laser cut or stamped and thenfolded from sheet metal such as copper, aluminium or steel.

As has already been stated in the description of FIG. 1 the invention isnot limited to the measurement of linear displacement. This will beobvious to those skilled in the art. If the arrangement of theelectrical intermediate device [2] and antenna [1] is bent around anaxis it can be seen that the invention is suitable for the measurementof rotary position. Further the electrical intermediate device andantenna can be arranged for a wide variety of sensing geometriesincluding curvilinear, 2-dimensional (by the use of multiple overlappinglinear encoders), cylindrical, roll, pitch and yaw.

To a significant extent, variation in the position of the electricalintermediate device [2] relative to the antenna [1] in axes other thanthe main measurement axis does not affect the measured value. Inparticular, the stand off distance between electrical intermediatedevice [2] and antenna [1] in the z-axis can vary without altering themeasured displacement. The range of acceptable variation can be extendedby bracketing the amplification factors used in the electronics circuit[18] according to the amplitude of the received signals. If theelectrical intermediate device [2] to antenna [1] distance is large thenthe amplitude of received signals will be small and large amplificationsshould be applied. The converse applies if electrical intermediatedevice [2] to antenna [1] distance is small. Since received amplitude isa coarse measure of z-axis separation then this can be used to call updifferent error correction factors stored within the electronics.

The invention is able to identify a multiplicity of electricalintermediate devices [2] and measure their displacement relative to anantenna [1] in a roughly concurrent fashion. This is accomplished byproviding each electrical intermediate device [2] with its own resonantfrequency. Individual resonant frequencies are most readily attained bycareful selection of different capacitor [7] values for example toproduce 3 electrical intermediate devices [2] with resonant frequenciesof 1, 2 and 3 MHz. The electronics circuit [18] can be programmed toexcite at these frequencies in turn and carry out measurements for eachelectrical intermediate device [2]. In order to maximise the frequencyand accuracy of measurements more sophisticated excitation andmeasurement algorithms can be used where, for example, the electricalintermediate device [2] which is found to be generally stationary ismeasured least and the one that has moved most recently or most often ismeasured most frequently.

Other frequencies not corresponding to resonant frequencies can be usedto measure, and hence cancel out, background received levels.

In order to maximise received signal strength and hence maximise thesignal:noise ratio the electronics circuit [18] should include afrequency tuning circuit so that the exact resonant frequency of theelectrical intermediate device [2] or devices may be used as thetransmit frequency. In this way a maximum amount of transmitted energygoes in to producing electrical resonance of the electrical intermediatedevice [2] so that it, in turn, produces maximum electromagnetic signal.

It will be appreciated by those skilled in the art that the invention isnot restricted by a particular number or arrangement of windings andthat various permutations of number, spacing and arrangement arefeasible.

Multiple resonant circuits and multiple antennae may be constructed onthe same physical unit of printed circuit board by simply avoidingelectrical connection between the various systems. Multiple layer PCB isparticularly useful in this regard. Such constructions are particularlyadvantageous in detectors for safety related environments whereelectrical redundancy is necessary. In an electrically redundant systema multiplicity of circuits of varying frequencies may be concurrentlydetected using a multiplicity of antennae energised with the relevantfrequencies, each antenna [1] being controlled by its own electroniccircuit [18].

In some safety related applications or applications where high levels ofavailability and reliability are required then detectors may beconstructed with self checking hardware and software. Not only can theelectronic circuit's [18] software contain the traditional techniquesassociated with checking for open circuits, out of bounds measurementsetc. but an electrical intermediate device [2] may be fixed relative tothe antenna [1]. In this way a self diagnostic check may be carried outby measuring the position of the fixed electrical intermediate device.If the fixed device appears to have moved from its known position, pastpredefined limits, then this may be taken as a fault and an alarm orcorrective action taken accordingly.

The invention's electromagnetic emissions can be reduced by balancingeach loop of the transmit winding [1 c] with a counter wound loop awayfrom the electrical intermediate device [2]. The electromagneticimmunity of the electrical intermediate device [2] can be increased in asimilar way by using opposing loops away from the transmit winding [1c]. The invention otherwise has a naturally high electromagneticimmunity due to the counter wound receive loops [1 a & 1 b] which arebalanced to far field.

Preferably any material present between the electrical intermediatedevice [2] and antenna [1] is an insulator such as plastic, ceramic orwood. A metal barrier can be placed between the electrical intermediatedevice [2] and antenna [1] so long as the excitation or resonantfrequency is sufficiently low to permit the signals to carry through themetal's skin depth. If a metal barrier is essential then preferably themetal has a relatively low magnetic permeability such as non-magneticstainless steel (e.g. 316 grade stainless). A frequency of 40 kHz is,for example, sufficient to permit the transmission of signals through 2mm thick, non-magnetic 316 grade stainless steel sheet.

The invention is limited in the maximum stand off distance betweenelectrical intermediate device [2] and antenna [1] compared to otherinductive techniques. This can be used advantageously in theconstruction of the antenna [1] and electronics circuit [18] on amultilayer printed circuit board. The antenna [1] may be arranged on thefirst pair of relatively thin layers and the electronics circuit [18]placed on one or more thin layers separated from the first two by athicker layer. The dimension of the thick layer is at least as large asthe maximum dimension of the antenna's [1] sensing range. This enablesthe electronics circuit [18] to be sufficiently out of range so as notto interfere with the antenna's [1] electromagnetic field but allows acompact arrangement and minimum use of circuit board.

For most applications the effect of extreme or changing temperatureswill have negligible effect on measurement performance. In someapplications, however, very high accuracy measurement is required eventhough the operating temperature range or variation rate may be extreme.In such instances, the relatively small expansion or contraction of theantenna [1] or target [2] may lead to erroneous measurement. Suchtemperature effects may be counteracted by measuring the actualoperating temperature and modifying the measured position accordingly,i.e. reducing or increasing the measured value according to thetemperature. Temperature can be measured using a thermocouple orresistance device but preferably the resistance of one or more of theantenna windings [1 a, 1 b or 1 c] can be used to provide an indicationof temperature. Measurement of the winding resistance is preferable tomeasurement by a thermocouple because the windings provide a morerepresentative measurement due to their position along the measurementaxis (rather than being constrained to a single point as with athermocouple).

There are many applications for the invention including, but not limitedto: actuators, aileron controls, angle sensors, radar antenna tracking,anti-counterfeit devices, audio controls, automatic guided vehicles,automatic teller machines, automation equipment, ball screws, boilers,brake sensors, brake wear sensors, burners, climate controls, cockpitcontrols, component identification, consumer electronics, cookers,cooking ranges, cooktops, dials, direction indicators, dishwashers,displacement sensors, door travel sensors, elevators, end of shaftencoders, fitness equipment, flow sensors, food mixers, fuel levelsensors, fuel metering, games, gauges, giant magnetoresistive sensorreplacements, guided vehicle tracking, gunnery sights, Hall affectreplacements, headlamp level controls, HVAC sensors, hydraulicactuators, hydraulic valves, identification tags, impellers,inclinometers, indexing tables, indicator gauges, Inductosynreplacements, industrial control panels, joysticks, kitchen goods,lifts, lighting controls, limit switch replacements, linear actuators,liquid level sensors, load sensors, LVDT replacements, machine tools,magnetostrictive sensor replacements, marine engines, marine equipment,mining equipment, missile guidance, motion controllers, motor encoders,odometers, packaging equipment, palletisers, paper thickness sensors,pedal sensors, pen sensing, petrochemical sensors, plotter controls,pneumatic actuators, pneumatic valves, pressure sensors, printer writeheads, PRNDL sensors, proximity sensors, push buttons, radar controls,ride height sensors, robots, roll/pitch/yaw sensors, roller separationsensors, rotary encoders, RVDT replacements, safety switches, seatinginstrumentation, security tags, servo motors, shaft encoders, sheetfeeders, skis, sliders, speed sensors, sports equipment, steering anglesensor, steering column controls, stepper motors, strain measurement,suspension dampers, suspension sensors, tachometers, tamper evidentdevices, throttle controls, tilt sensors, torque sensors, toys, tractioncontrol, transmission sensors, turbines, user interface elements,utility meters, valves, velocity sensors, vibration sensors, washingmachines, weight sensors, wheel sensors, workpiece identification.

1. An inductive position detector comprising a first inductive deviceand a second inductive device which defines a measurement path andcomprises at least two receive windings, wherein: relative movement ofthe first and second inductive devices changes the position of the firstinductive device on the measurement path; the first inductive devicecomprises a substantially laminar passive resonant circuit formed by aninductive winding in series with a capacitor, with the inductive windingforming a loop whose extent in a direction transverse to the measurementpath varies periodically; the receive windings of the second inductivedevice are each formed by at least a respective outward conductivewinding in series with a respective return conductive winding; theoutward winding and the return winding of each receive winding cross atleast three times to form at least four inductive loops where adjacentloops have opposite current flow, in use, and the transverse distance ofeach outward winding to the measurement path varies periodically withdistance along the measurement path; and the second inductive device isconfigured to induce an alternating current in the passive resonantcircuit, in use, such that the induced alternating current in thepassive resonant circuit induces an alternating signal in each receivewinding by virtue of the mutual inductance of the receive windings andthe passive resonant circuit, which signals are indicative of theposition of the first inductive device on the measurement path.
 2. Aninductive position detector as claimed in claim 1, wherein thetransverse distance of the return winding of each receive winding to themeasurement path varies periodically with distance along the measurementpath.
 3. An inductive position detector as claimed in claim 1, whereinthe area defined by each of the said inductive loops is substantiallyequal.
 4. An inductive position detector as claimed in claim 1,comprising a plurality of first inductive devices, each comprising arespective resonant circuit, with each resonant circuit having adifferent resonant frequency.
 5. An inductive position detector asclaimed in claim 1, wherein at least one receive winding of the secondinductive device is supplied, in use, with a pulsed alternating currentwhich induces an alternating current in the passive resonant circuit,such that the passive resonant circuit induces an alternating current inthe receive windings of the second inductive device in the off periodsof the pulsed alternating current.
 6. An inductive position detector asclaimed in claim 1, wherein the second inductive device comprises atransmit winding arranged to induce an alternating current in thepassive resonant circuit.
 7. An inductive position detector as claimedin claim 6, wherein the transmit winding has a substantially constantwidth along the measurement path.
 8. An inductive position detector asclaimed in claim 1, wherein the shape of the periodic variation of thetransverse distance of the outward winding to the measurement path withdistance along the measurement path is rectangular, sinusoidal,triangular, hexagonal or polygonal.
 9. An inductive position detector asclaimed in claim 1, wherein the period of the variation of the extent ofthe inductive winding of the first inductive device in the directiontransverse to the measurement path is the same as that for the variationof the transverse distance of the outward winding of each receivewinding.
 10. An inductive position detector as claimed in claim 1,wherein the period of the periodic variation for each outward conductivewinding is the same and the second receive winding is displaced relativeto the first receive winding by a fixed distance substantially equal toan integer multiple of one quarter of the period.
 11. An inductiveposition detector as claimed in claim 1, wherein the period of theperiodic variation for each outward conductive winding is different. 12.An inductive position detector as claimed in claim 11, wherein thedifference in the periods of the two outward windings is such that thecombination of the distance of each outward winding from the measurementpath at a given position along the measurement path uniquely identifiesthat position along the measurement path.
 13. An inductive positiondetector as claimed in claim 1, wherein the two outward windings arespaced in a direction transverse to the measurement path.
 14. Aninductive position detector as claimed in claim 1 further comprising asignal processor configured to generate a speed signal by processing theinduced signal in at least one of the receive windings and furtherconfigured to generate a position signal by comparison of the inducedsignal in the two receive windings.
 15. An inductive position detectoras claimed in claim 1, wherein the windings are provided as conductivetracks on a printed circuit board.
 16. An inductive position detector asclaimed in claim 1, wherein the windings of the second inductive deviceare provided on a substantially laminar substrate having two opposedfaces and for each receive winding the outward conductive winding isprovided on one of said faces and the return conductive winding isprovided on the other of said faces.
 17. An inductive position detectoras claimed in claim 1, wherein the measurement path is linear.
 18. Aninductive position detector as claimed in claim 1, wherein themeasurement path is curved, particularly circular.
 19. A method oftemperature compensation in a detector as claimed in claim 1, the methodcomprising measuring the resistance of a winding of the second inductivedevice.
 20. A method of measuring torque on a rotating shaft, the methodcomprising: providing an inductive position detector as claimed in claim1; mounting the first inductive device about the shaft for rotationtherewith; mounting the second inductive device on the shaft forrotation therewith such that measurement path follows thecircumferential direction of the shaft; detecting the induced signal inthe second inductive device by means of inductive coupling; determiningrelative movement of the first and second inductive devices indicativeof a torque on the shaft.